Method and device for partial electrochemical treatment of bar-shaped objects

ABSTRACT

In a device, rod-shaped objects  10  (rods of various length and diameter) are electrochemically partially processed in dip plants (galvanized, pickled). Stationary tubular electrodes  30 , into which the rods  10  are centrically entered, are provided in the plating tank of the plant. The surfaces to be plated are each axially limited by an adjustable membrane carrier  26 , in which elastic shielding membranes  9  for delimiting the field lines are arranged. The membranes  9  are held by membrane holders  5 . The membrane holders  5  are arranged in a cage where they are free to move radially and are provided with inner centering springs  7  and with outer centering springs  19  so that the membrane holder  5  and the membranes  9  are held in such a way as to be self-centering.

The invention relates to a device as well as to a method for the partialprocessing, e.g., galvaniz galvanizing, electrochemical pickling orelectrochemical cleaning of bar-shaped objects in dip plants, namely ingalvanizing lines, pickle lines and cleaning lines.

In the following description, the objects to be processed will bereferred to as the rod. Devices for electrochemical pickling,galvanizing and electrochemical cleaning of long stretched-outcylinders, in particular of round rods, are well known. At one end or atboth, these rods may be tapered and/or threaded. One such example is thecylinder in shock absorbers in vehicles. The invention however is notlimited to the processing of round rods. It is also suited for thetreatment of rods having another section.

The electrochemical processing of the rods serves for example to improvethe wear and corrosion resistance properties of shock absorbercylinders. To this purpose, the rods are electrolytically plated withhard-chromium at only such surfaces which are subjected to load duringoperation. The other places of the rods are intended to remain uncoatedor to be partially provided with a thin layer of flash chrome fortemporarily protecting their surface. In order to improve the adherenceof the chromium layer, the surface is electrochemically pickled first.Insoluble electrodes are preferably employed for either step in theprocess. Between the various steps of the treatment, the rods arecleaned in galvanizing lines. A conveying facility brings the rods fromone processing station to the other.

To partially process the rods in their central region, the ends have tobe masked, i.e., shielded in such a way that no metal or, as a temporaryprotection flash chromium, can be deposited onto these dimensionallyaccurately beforehand defined areas. The boundaries between the area ofdeposition and the areas not to be coated generally have closetolerances. On certain shock absorber cylinders, the uncoated area mustverge into the coated area e.g. within a range of ±1 millimeter only.The thickness of the coating to be deposited must be uniform up to thisboundary. Edge effects, i.e., an increase or decrease of the coating'sthickness on its boundaries must be avoided. The goal of such anaccurate coating is to eliminate the need for subsequent polishing.

In the plants of the art, the rods are attached to racks where they areprocessed, the proper racks being attached to movable flight bars. Theracks are provided with individual masks in such a manner that the twoends of the rod are not coated in the predetermined region. In anelectroplating plant, masks and racks must be available in sufficientnumber for all the current rod lengths, diameters and areas of plating.Since the racks also serve to feed the current conducted by the bath tothe rods, they have to be made of a conductive metal. Prior togalvanizing, the metal has to be protected by layers of plastic.

The document DE-AS 11 03 103 describes a device for galvanic chromiumplating of the outer surface of disk valve stems. In order to permitpartial galvanizing of the stems, said stems are accommodated in acentric manner in tubular anodes and are fastened by the upper side in astepped metallic core. Current is fed via the metallic core, themetallic core being separated from the electrolytic cell by aninsulating body. The metallic core and the insulating body constitutethe boundary for the galvanic coating at the upper end of the stem. Thelower end is protected against undesired galvanizing by a protectivelacquer.

The disadvantage of this embodiment is that each body to be galvanizedhas to be attached individually and that the protective lacquer has tobe applied with dimensional accuracy to one end of the body. To removethe protective lacquer once galvanizing has been completed is alsocomplicated.

DE 197 22 983 A1 indicates a method and a device permitting to partiallyelectrochemically process, in particular to electrochemically processrods of various dimension in dip plants.

For each rod, defined adjustable masks with sealing means in the form ofterminal collars are employed at either end of the rod to delimit thesurfaces to be treated. In a charging station, several electricallyconductive grippers, which are attached to the flight bar,simultaneously grasp one rod each on one side. By immersing the rodsinto the electrolytic processing station, each rod enters an individualcell constituted by the rod and one stationary tubular electrode. Eitherof the electrolytically effective upper and lower end of the electrodeis defined by a tubular, axially adjustable mask. Each end of the maskis terminated by a collar. By immersing the flight bar, the rod firstpasses through the upper collar, enters the electrode and eventuallytraverses the lower collar. By displacing the upper and lower maskindependently from one another parallel to the axis of the rod, the twoplating boundaries, or the surface on the rod which is to be plated, areuniformly adjusted for each flight bar or for each row of rods on theflight bar. This method overcomes the drawbacks of the prior art racktechnique, in which for each rod dimension a special mask in the form ofcaps or holders is needed. In practical operation however, the objectsto be processed are often hanging crookedly since metal has deposited onthe tongs holders or since the holders are damaged. When the no longercentrically hanging objects to be processed are being entered into themasks, the collars laterally gape and may be damaged by sharp edges. Asa result of the damaged collars, accurately dimensioned galvanizing isno longer possible. What makes it even more difficult is that damages onthe collars only become noticeable once the galvanized articles haveexited the plant, since the collars, when immersed in the electrolyte,cannot be seen from the surface of the bath. The articles processedunder these conditions are unserviceable.

The basic problem of the present invention is therefore to avoid thedrawbacks of the methods and devices of the art for shielding rodsduring the galvanizing process and in particular to keep the amount ofwork required for shielding such areas on the rods that are not to begalvanized as small as possible. Moreover, the method is intended towork perfectly, even under manufacturing conditions.

The solution of this problem is given by a self-centering deviceaccording to the instant invention.

The device according to the invention serves for the partialelectrochemical processing of rod-shaped objects in dip plants. Thedevice comprises

a. at least one plating tank,

b. in the at least one plating tank, tubular electrodes and at least onetubular membrane carrier in which the objects to be processed may atleast partially be entered, and

c. membrane holders arranged within the at least one membrane carrier.

The tubular membrane carriers are preferably made of a chemically stablematerial which is electrically nonconductive on its surface at least.

According to the dimensions of the objects to be processed, the deviceis provided with axially adjustable, electrically nonconductiveshielding facilities. At least one shielding facility is provided foreach rod. A shielding facility may be provided for example at the lowerend of the rod within the plating tank in order to prevent this portionof the rod from being electrochemically treated. In this case, the upperportion of the rod could remain untreated by not submerging it into thesolution. Shielding facilities may also be provided at either end of therod, though.

The shielding facilities comprise at least one cage (34) within thetubular membrane carrier. The cages are arranged in such a manner thatthe objects to be processed are capable of being pushed through them andare each formed by at least one cage cover and at least one cage bottom,respectively. A membrane holder holding a membrane is carried in aradially movable fashion between the cage cover and the cage bottom ineach cage. At least one inner centering spring for guiding the objectsto be processed and at least one outer centering spring for centeringthe membrane holder and the membrane in the cage are provided on eachmembrane holder. The membranes are preferably made of a chemicallystable, extensible and electrically nonconductive material.

The cages are arranged on that side of the shielding facility which isfacing the center of the rod.

The outer centering springs are preferably rod-shaped and designed asleaf springs and are tangentially fastened to the outer surfaces of themembrane holder. Their function is to center the membrane holder and themembrane in the cage and they oppose a radial pressure exerted by anoff-center rod onto the membrane and the membrane holder. The force ofthe outer centering springs is adapted to the tensile property of theelastic membranes. It should be selected to be so large as to preventthe membranes from gaping as a result of lateral pressure applied by theobjects to be processed or, in the extreme, from tearing, thusdestroying the shielding effect at this place.

The force of the inner centering springs should however be selected tobe smaller than the spring force of the holding tongs for the rods inorder not to jeopardize the secure fastening of the rods on and theirelectrical contact with the holding tongs. If the lateral pressure ofthe rods which are received by the inner springs of the membrane holderis too high, i.e., the centering effect is no longer sufficient tocenter the rods, the membranes are pushed off-center while the rods areentering the shielding facility, thus preventing the membranes frombeing damaged. In so doing, the accurate shielding of the field lines isnevertheless made possible. By contrast, in the shielding membranes ofthe art which are rigidly fastened to the shielding facility, themembranes made of elastic material gape upon lateral pressure exerted bythe rods so that the shielding action is impaired at this place.

Within the lower membrane carrier and underneath the membrane, there aremounted, acting as inner centering springs, either several wings ofcentering springs which are tapering toward the center and areresiliently attached to the membrane holder or at least three resilientcentrically arranged spring rods that constitute a guide, said innercentering springs having a centering effect on the rods by gripping therods threaded through the membrane, thus acting as a centering guide.More specifically, the spring bars may be given an elongated shape andmay, starting from a plane formed by the membrane holder and runningslantways, form together with the free ends, an opening which is alignedwith the center of the membrane holder, and through which the rods canbe passed.

At least two non-rotatable spring bars attached at one end to themembrane holder and having mating non-rotatable centering guidesattached to the free end of the membrane holder may be utilized as innercentering springs within the lower membrane carrier. Said spring barsare oriented in such a way that a rod, which may be threaded through themembrane held by the membrane holder, may be gripped by the spring bars.The guides provided for the rods are each prismatic and essentiallyparallel to the spring bars.

The inner centering springs accommodated within the upper membranecarrier may specifically be of an elongated shape and may, standingessentially upright on a plane constituted by the membrane holder, form,together with the free ends which are bent at an angle of at least 45°0,preferably of at least 90° and in particular of at least 180°, anopening which is aligned with the center of the membrane holder andthrough which the rods can be passed.

Upon exiting the rods out of the shielding device of the invention, theouter springs on the membrane holders ensure that the membranes centerthemselves again for the next batch of rods entering the device, i.e.,that they return to their original position.

In order to adjust the device for the processing of various rod lengthsor of various geometrical shapes of the rod areas to be plated and notto be plated, height adjusting facilities are provided by means of whichthe membrane carriers with the membrane holders and the membranes may bemoved in vertical direction.

The device permits to conduct a method for partially electrochemicallyprocessing the rods in dip plants involving the following stages:

a. Grasping the essentially vertically oriented rods by means ofappropriate holding elements;

b. Immersing the rods into a plating tank and into a membrane holderarranged within at least one membrane carrier, the plating tank beingprovided with tubular electrodes and with at least tubular one membranecarrier into which the rods may be at least partially entered;

c. Conveying the objects to be plated through one central opening eachin at least one membrane held by the membrane holders.

In the process of passing through the at least one membrane, such rodswhich are not vertically arranged and the central openings in themembranes are automatically aligned.

The invention will be described in more detail with the help of theexamples illustrated in the Figures listed below. BRIEF DESCRIPTION OFDRAWINGS

FIG. 1a shows in schematic form a section through a charging station ofthe art;

FIG. 1b shows in schematic form a section through a rigid mask in acharging station of the art;

FIG. 2 shows in schematic form a section through an upper membranecarrier of the invention;

FIG. 3 shows in schematic form a section through a lower membranecarrier of the invention;

FIG. 4a shows in schematic form an embodiment of a lower membraneholder;

FIG. 4b shows in schematic form another embodiment of a lower membraneholder.

DETAILED DESCRIPTION OF DRAWINGS

FIG. 1a shows a vertical section through a plating tank with severalconventional electrolytic individual cells for the chromium plating ofrods. FIG. 1b shows a partial vertical section of a single conventionalelectrolytic individual cell inside the plating tank of FIG. 1a.

A plating tank 14 is filled up to the rim 33 with an electrolyte.Tubular electrodes 30 which are connected in an electric conductive wayto a source of direct current (not shown) are mounted in the platingtank. Furthermore, two diaphragm carriers 17 and 25 which are adjustablein vertical direction independently from one another are provided in andat the plating tank 14. On the flight bar 1, which is kept in the rightposition in the bath by way of the support for the flight bar 2, thereare arranged holding tongs 3 which make electrical contact with the rods10 and hold them fast. On lowering the flight bar 1 into the tubularelectrodes 30, the rods 10 first pierce the collars 27 in the uppermembrane carrier 26 which are rigidly attached to the verticallyadjustable diaphragm carrier 25 (FIG. 1b). On further lowering theflight bar 1, the rods 10 also pierce the collar 24 of the lowermembrane carrier 23 (FIG. 1b), which is attached to the lower diaphragmcarrier 17. The lower diaphragm carrier 17 with the membrane carrier 25and the collars 24 (FIG. 1b) as well as the upper diaphragm carrier 23with the membrane carrier 26 and the collars 27 are adjustable in heightthanks to the adjusting facilities 20 and 28 in conformity with therequirements of the rods 10. In order to further reduce the currentdensity in the lower area of the rods, conductive auxiliary cathodes 22may be provided as shown in FIG. 1b, said auxiliary cathodes being urgedagainst the immersed objects 10 by way of springs 21, thus considerablyincreasing the cathodic acting surface or reducing the cathodic currentdensity so much that no deposit can henceforth take place in the lowershielded area In this way, rods 10 with various dimensions and varyingrequirements for the position of the partial plating/processing can beprocessed in the tank.

In the following, reference will be made to the embodiments inaccordance with the invention. As far as specific elements of the deviceare not illustrated in the FIGS. 2 through 4, like elements will insteadbe referred to with the corresponding reference numerals in the FIGS. 1aand 1 b. So far, the FIGS. 1a and 1 b also illustrate inventive featuresof the device.

In FIG. 2, the upper membrane carrier 26 according to the invention isillustrated in a vertical sectional drawing as a cutout of an individualgalvanizing cell inside the plating tank 14 which is not illustratedherein (see FIG. 1a). The tubular membrane carrier 26 is attached to theupper diaphragm carrier 25, which is adjusted by way of a heightadjusting facility 28 not shown (see FIG. 1a) to the position suited forthe rods to be produced.

The adjustment of the diaphragm carrier 25 concurrently causes theelectric nonconductive tube of the membrane carrier 26 to penetrate theinsoluble electrode 30 which is shown in a cutout, thus shielding thatlength of the electrode, which is not needed. On immersing the rods 10into the plating tank 14, the diaphragm carrier 25 may advantageously beplaced in the upper position.

The diaphragm carrier 25 and all of the elements that are submerged intothe plating tank are made of a chemically stable material. PVC, PVDF,PTFE for example are available as electric nonconductive materials. Forelectric conductive materials, titanium or lead may be used.

When the flight bars 1, which is not illustrated herein, is immersed,the rod 10 is first entered into the area of the inner centering springs7 which apply a centering pressure to the rod 10 and align it, when itis not hanging centrically downward If the pressure of the spring is notstrong enough to align the rod 10, the membrane holder 5, which is heldby the outer centering springs 19, is shifted in horizontal direction,so to adapt to the position of the rod in order to prevent the delicatemembrane 9 from being damaged or expanded. In order to make certain ofthe functioning of the membrane holder 5, the cage cover 12 and the cagebottom 16 are accommodated on the tubular membrane carrier 26 in such amanner that the membrane holder 5 is free to move in horizontal (radial)direction, but is sufficiently held fast in vertical direction. Afterthe rod 10 has been lowered to its lowermost position, the upperdiaphragm carrier 25 may be brought downward into the shielding positionprescribed by the design of the rod. In a preferred embodiment, theshielding membrane 9, which sits close to the rod 10, in the processarches from the inside in a conical shape upward. This reduces the“bone” effect (the formation of a thickened galvanic deposit) at theends of the rod 10.

Upon completion of the treatment, the rod 10 is vertically exited upwardout of the galvanizing cell. As soon as the rod 10 has exited themembrane 9, the membrane holder 5 is centered again by way of the outersprings 19 in the center of the membrane carrier 26.

In FIG. 3, the lower membrane carrier 23 according to the invention,together with a small cutout of the diaphragm carrier 17, is shown in avertical section through an individual galvanizing cell inside the notshown plating tank 14 (see FIG. 1a). The tubular membrane carrier 23 isattached to the lower diaphragm carrier 17 and may be brought by way ofthe height adjusting facility 20 which is not illustrated herein (seeFIG. 1a) into the lower position suited for the rods 10 to be produced.The height of the membrane carrier 23 is preferably adjusted before thenot shown flight bar 1 is immersed.

In principle, the structure of the lower shielding facility is equal tothe upper shielding facility of FIG. 2. Here too, the membrane holder 4with the outer centering springs 18, the cage cover 11, the cage bottom15 as well as the inner centering springs 6 are to be found. The innercentering springs 6 however are arranged underneath the membrane 8 inorder to prevent them from having a disturbing influence upon theconcentration of the field lines in the galvanizing area of the rod 10.It is important in this case that the distance chosen between thecentering spring 6 and the membrane 8 is not too great in order toprevent the membrane 8 from being damaged by the not yet centered rod 10or the right position of the membrane 8 from being achieved. After therod 10 has passed through the upper shielding facility while beingsubmerged into the cell, it impinges first on the membrane 8 as it isfurther immersed, is then passed through said membrane, is moved againstthe inner centering spring 6 and is centered if necessary (when thecentering action of the upper shielding facility is not sufficient). Ifthe pressure upon the spring 6 is too great, the membrane holder 4 isagain horizontally (radially) shifted together with the membrane 8against the spring force of the outer centering springs 18 in order toprevent the membrane 8 from gaping at the side which is not compressedby the rod 10 or from being damaged. In this way, it is guaranteed thatthe end of the rod is shielded with accuracy at its border. In just thesame way as with the upper shielding facility, the outer centeringsprings 18 see to it that, after the rods 10 have exited the lowershielding facility, the membrane holder 4 returns to its originalposition, i.e., that it centers itself anew.

Depending upon the requirements of the rods 10, an upper diaphragmingdevice only, a lower diaphragming device only, or both (upper and lower)can be used. A combination of the shielding facilities top/bottom islikewise possible in accordance with the invention and the state of theart.

FIG. 4a shows a top view and a side view of a preferred embodiment of alower membrane holder 4. The outer bar-shaped centering springs 18 aretangentially accommodated at the outer circumference of the cylindricalmembrane holder 4. The inner centering springs 29 are wing-shaped andare staggered or inclined toward the center of the membrane holder 4, sothat three conically running surfaces are created inside, against whichthe rod 10 that is not hanging centrically (see FIG. 3) is pressed whenbeing entered into the facility. On further lowering the rod 10, thewing-shaped centering springs 29 flex downward and outward. Thisspecific embodiment has the advantage that, on account of the reduceddemands on the properties of the spring, abrasion resistant plastics maybe used as a material with ideal shielding properties for the innercentering springs 29.

FIG. 4b again shows a membrane holder 4. As compared with the membraneholder 4 of FIG. 4a, the inner centering springs are provided in anotherform. Here, the centering springs consist of one spring bar 32 each,said spring bar being non-rotatably attached to the outer side of themembrane holder 4. A prismatic guide 31 is accommodated at the innerside of the spring bar 32, said guide being secured against rotation aswell. The mounting direction of the spring bar 32 points diagonallydownward starting from the membrane holder. Both springs 32, inconjunction with the prismatic guides 31, bring about a good centeringof the rods 10 (see FIG. 3).

The inner centering springs 29, 32 of the FIGS. 4a and 4 b are alsosuited for use on the upper membrane holders 5. In this case though, thecentering springs 29, 32 must be arranged above membrane holder 5 asshown in FIG. 2.

LISTING OF REFERENCE NUMERALS

1 flight bar

2 support for the flight bar

3 holding tongs

4 membrane holder bottom

5 membrane holder top

6 inner centering spring bottom

7 inner centering spring top

8 shielding membrane bottom

9 shielding membrane top

10 rod (objects)

11 cage cover in lower shielding facility

12 cage cover in upper shielding facility

13 plastic plate

14 plating tank

15 cage bottom in lower shielding facility

16 cage bottom in upper shielding facility

17 lower diaphragm carrier

18 outer centering spring for shielding membrane bottom

19 outer centering spring for shielding membrane top

20 height adjusting facility for shielding facility bottom

21 pressure spring

22 auxiliary cathode

23 tubular membrane carrier bottom

24 collar bottom

25 upper diaphragm carrier

26 tubular membrane carrier top

27 collar top

28 height adjusting facility for shielding facility top

29 wing-shaped centering spring

30 electrode

31 prismatic guide

32 spring bar

33 rim

What is claimed is:
 1. Device for the partial electrochemical treatmentof rod-shaped objects in dip plants, comprising a. at least one platingtank (14), b. in the at least one plating tank (14), tubular electrodes(30) and at least one tubular membrane carrier (23, 26) in which objectsto be processed may at least partially be entered, and c. membraneholders (4, 5) arranged within the at least one tubular membrane carrier(23, 26) wherein at least one cage (34) is provided within the at leastone tubular membrane carrier (23, 26), said cage being arranged in sucha manner that the objects (10) to be processed are capable of beingpushed there through and each said cage being formed by at least onecage cover (11, 12) and at least one cage bottom (15, 16), a membraneholder (4, 5) holding a membrane (8, 9) being carried in a radiallymovable fashion between the cage cover (11, 12) and the cage bottom (15,16) in each cage and wherein at least one inner centering spring (6, 7,29, 32) for guiding the objects (10) to be processed and at least oneouter centering spring (18, 19) for centering the membrane holder (4, 5)with the membrane (8, 9) are provided in the cage.
 2. Device accordingto claim 1, wherein at least three inner centering springs (6, 7) areattached to a membrane holder (4, 5) and are oriented in such a mannerthat objects (10) that may be pushed through the membrane (8, 9), whichis held by the membrane holder (4, 5), may be gripped by the springs (6,7) and wherein the springs (6, 7) constitute a centering guide for theobjects (10).
 3. Device according to claim 1, wherein at least twospring bars (32) are provided as inner centering springs, said springbars being non-rotatably attached by their one end to the membraneholder (4, 5) and being oriented in such a manner that objects (10) thatmay be pushed through the membrane (8, 9), which is held by the membraneholder (4, 5), may be gripped by the spring bars (32), and prismaticguides (31) for the objects (10), which are running essentially parallelto the spring bars (32), being non-rotatably attached to their free endsthus allowing the objects (10) to be centered by said guides.
 4. Deviceaccording to claim 1, wherein wing-shaped springs (29) are provided asinner centering springs, said wing-shaped springs being attached bytheir one end to the membrane holder (4, 5) and being oriented in such amanner that objects (10) that may be pushed through the membrane (8, 9),which is held by the membrane holder (4, 5), may be gripped by thesprings (29), the springs (29) constituting together a centering guidefor the objects (10).
 5. Device according to claim 4, wherein the innercentering springs (6, 7) have an elongated shape and form, together withthe free ends, starting from a plane formed by the membrane holder (4,5) and running slantways, an opening which is aligned with the center ofthe membrane holder (4, 5), and through which the objects can be passed.6. Device according to claim 4, wherein the inner centering springs (6,7) have an elongated shape and, standing essentially upright on a planeconstituted by the membrane holder (4, 5), form together with the freeends which are bent at an angle of at least 45°, an opening which isaligned with the center of the membrane holder (4, 5) and through whichthe objects can be passed.
 7. Device according to one of the previousclaims 1-6, wherein the leaf springs (18, 19) acting as outer centeringsprings (18, 19) are tangentially attached to the membrane holder (4,5).
 8. Device according to one of the previous claims 1-6, wherein thetubular membrane carriers (23, 26) are made of a chemically stable andat least on its surface electric nonconductive material.
 9. Deviceaccording to one of the previous claims 1-6, wherein the membranes (8,9) are made of a chemically stable, expandable and electricnonconductive material.
 10. Device according to one of the previousclaims 1-6, wherein the membrane carriers (23, 26) together with themembrane holders (4, 5) and the membranes (8, 9) are movable in verticaldirection by way of height adjusting facilities (20, 28).
 11. Processfor partially electrochemically processing rod-shaped objects in dipplants comprising the following steps: a. Grasping the essentiallyvertically oriented objects by means of appropriate holding elements; b.Immersing the objects into a plating tank and into a membrane holderarranged within at least one membrane carrier, the plating tankcomprising with tubular electrodes and at least one membrane carrierinto which the objects may be at least partially entered; c. Conveyingthe objects to be plated through one central opening each in at leastone membrane held by the membrane holders; wherein, in the process ofpassing through the at least one membrane (8, 9), such objects which arenot vertically arranged and the central openings are automaticallyaligned.